Elastin is the chief elastic component of connective and supporting tissues including ligaments and the walls of major blood vessels. It is found in association with other structural proteins; in particular the elastic fiber contains a glycoprotein that is a microfibrillar element. Although fibroblasts have been considered as the generic cell type for biosynthesis of elastin, this conclusion remains primarily speculative and inferential; more sound histological evidence exists that smooth muscle cells may produce elastin. The molecular properties of elastin responsible for its elastic capacity are: (a) a polypeptide backbone with perhaps less than 10 per cent of helix; (b) sidechains that are largely non-polar or hydrophobic that give the polypeptide chains a "rubberlike" character; and (c) occurrence of crosslinking amino acids (desmosine, isodesmosine and precursors of these, lysinonorleucine, and the aldol of gamma-aminoadipic-delta-semialdehyde); these allow for elastic return. Until about ten years ago, elastin because of its patent insolubility as a fiber and its close association with collagen and carbohydrate components, was not readily amenable to investigation. A soluble sort of "pathological" precursor elastin, non-crosslinked, can now be produced by making animals deficient in copper. The present program is concerned with the chemical and antigenic structure of elastin, its biosynthesis, its biological transformations in development and perhaps in disease, its association with other tissue components, and its degradation by elastases. Specifically we are interested in developing a series of elastolytic procedures for the degradation of elastin to yield fingerprinting peptide maps; the localization of specific antigenic determinants in the so-called precursor elastins; the influence of formation of crosslinks during development on provision of new antigenic determinants; and, by use of the enzymatic, chemical and immunologic information so obtained, the determination of the site of nascent elastin in fibroblasts and in smooth muscle cells.